Depletion of human peripheral blood leukocytes (PBL) of Leu M3+ cells by adherence to plastic and Sephadex G10 results in the abrogation of proliferative (3H) and cytotoxic T lymphocyte (CTL) responses to influenza A virus (FLU), but in an elevation of H and CTL responses to HLA alloantigens (ALLO). This loss of the FLU T cell response was attributed to removal of antigen-presenting cells (APC), whereas the elevation of the ALLO response was shown to be due to a suppressor cell (or a suppressor inducer cell) that is contained in the Leu M3+ adherent cell population. Suppressor activity was inactivated by culturing either unfractionated PBL or adherent cells with viruses, including influenza A, adenovirus or vaccinia virus. The suppressor activity was attributed to a soluble factor produced by Leu M3+ cells.